1. Field of the Invention
The present invention relates to a furnace for forming an optical fiber which heats, melts, and draws an optical fiber preform to produce an optical fiber (optical fiber drawing furnace), more particularly an optical fiber drawing furnace provided with a means for preventing entry of ambient air from the outside of the optical fiber drawing furnace at a bottom thereof.
2. Description of the Related Art
There is strong demand for the improvement of the productivity of optical fibers due to the rapid increase in applications of optical fibers. To satisfy this demand, attempts are being made to increase the diameter of the optical fiber preform to be drawn and to raise the drawing speed. The high speed drawing of an optical fiber preform having a large diameter, however, causes the pressure in an inner space of an optical fiber drawing furnace to vary more due to the vibration of the moving optical fiber preform having the large diameter which in turn causes variations in the drawing speed of the optical fiber and variations in the diameter of the drawn optical fiber. Sometimes, the pressure in the inner space of the optical fiber drawing furnace becomes lower than the atmospheric pressure (pressure outside optical fiber drawing furnace). If the pressure in the inner space of the optical fiber drawing furnace is lower than that of the atmosphere, air may invade the inner space to contact the surface of the optical fiber preform, the surface of the drawn optical fiber, and the inner surface of a furnace tube arranged in the optical fiber drawing furnace. The contact of the surfaces of the optical fiber preform and the optical fiber by air results in deterioration of the quality of the drawn optical fiber and weakens the mechanical strength of the drawn fiber. The contact of the inner surface of the furnace tube with air shortens the life of the tube. Accordingly, a variety of improvements have been experimented with.
One approach for overcoming the above problem is to increase the amount of inert gas which is introduced and filled in the inner space of the optical fiber drawing furnace. The increase of the amount of inert gas introduced into the inner space, however, lowers the temperature of the optical fiber preform and makes it necessary to raise the heating temperature of the heater to maintain a suitable temperature for drawing the optical fiber in the inner space. Since the furnace tube is usually made of carbon, the raise of the heating temperature may accelerate the deterioration of the furnace tube. The rapid deterioration of the furnace tube causes the rapid generation of carbon duct which in turn reduces the quality and strength of the optical fiber. The rapid deterioration of the furnace tube, i.e., the insufficient life of the furnace tube, therefore makes it necessary to frequently replace the furnace tube, resulting in troublesome maintenance and increased production costs.
Other countermeasures have been devised as well. FIG. 1 is a sectional view of an optical fiber drawing furnace 101 of the related art.
The optical fiber drawing furnace 101 includes a furnace body 104, a heater 103 for heating and melting an optical fiber preform 105 introduced into an inner space 108 of the furnace body 104 from the top of the furnace body 104, a furnace tube 102 arranged at an inner wall of the heater 103, a gas introducing portion 107 for introducing inert gas such as argon (Ar) gas or helium (He) gas into the inner space 108 from the lower portion of the furnace body 104, and a bottom cover 109. The bottom cover 109 has a hole (aperture) through which an optical fiber 106 is drawn to the outside of the furnace body 104.
The furnace tube 102 is made of, for example, carbon and is arranged in the inner wall of the furnace body 104 at a position at which the heater 103 is provided to prevent direct contact between the heater 103 and the optical fiber preform 105.
In the optical fiber drawing furnace 101, the optical fiber preform 105 is introduced into the inner space 108 of the furnace body 104, heated and melted by heat from the heater 103, and pulled down at a predetermined tension to form the optical fiber 106. The optical fiber 106 is extracted through the hole formed in the bottom cover 109.
Usually, the optical fiber preform 105 is heated and melted at around 2000.degree. C. Thus, if the optical fiber 106 is formed in a normal atmosphere including oxygen, the carbon furnace tube 102 may be oxidized and damaged and dust may occur. The dust may lower the characteristics of the drawn optical fiber 106. The oxidization of the furnace tube 102 results in a shorter life of the furnace tube 102 and ends up raising the production costs of the optical fiber 106.
Therefore, the gas introduction portion 107 is provided at the lower portion of the furnace body 104 to introduce an inert gas such as argon (Ar) gas, nitrogen (Ni) gas, or helium (He) gas into the inner space 108 of the furnace body 104 so as to prevent entry of ambient gas, such as air, through the hole in the bottom cover 109. The pressure in the inner space 108 should be higher than the pressure of the ambient gas to prevent entry of ambient gas from the hole of the bottom cover 109. The inert gas introduced into the inner space 108 heads mainly to the top of the furnace body 104 and partially to the outside of the optical fiber drawing furnace through the hole of the bottom cover 109.
The hole in the bottom cover 109 must be a predetermined diameter so that the drawing optical fiber 106 can pass through it at a high speed without contacting the same. Accordingly, it is impossible to completely prevent entry of ambient gas through the hole of the bottom cover 109. To improve the extent to which entry of ambient gas is prevented, a higher pressure state of the inner space 108 and a larger amount of the introduction of the inert gas through the gas introduction portion 107 are necessary, but these lower the temperature in the inner space 108 and waste the inner gas, as discussed above.
Another related art will be described referring to FIG. 2.
An optical fiber drawing furnace 111 illustrated in FIG. 2 includes a furnace body 114, a heater 113, a furnace tube 112, a bottom cover 119 arranged at a bottom of the furnace body 114 and having a hole thorough which a drawing optical fiber 116 passes, and a gas introduction portion 117 provided at a lower portion of the furnace body 114 and immediately above the bottom cover 119. These structures are substantially identical to those in FIG. 1.
The optical fiber drawing furnace 111 illustrated in FIG. 2 further includes an additional gas introduction portion 120 at an upper portion of the furnace body 114 through which an optical fiber preform 115 is introduced into an inner space 118. Inert gas introduced through the additional gas introduction portion 120 prevents entry of the ambient gas to the inner space 118 from the top at which the optical fiber preform 115 is introduced into the inner space 118.
The inert gas is introduced into the inner space 118 through the gas introduction portion 117 to maintain a positive pressure state of the inner space with respect to the outside of the furnace 11 so as to prevent entry of the ambient gas into the inner space 118 through the hole of the bottom cover 119.
The optical fiber drawing furnace 111 suffers from the disadvantage of the entry of the ambient gas into the inner space 118 of the furnace body 114 through the hole in the bottom cover 119 due to the same reasons as to those described above with reference to FIG. 1. Thus, the optical fiber drawing furnace 111 still suffers from the disadvantages of the short life of the optical fiber drawing furnace 111, the low characteristics of the optical fiber 116, and the increased production cost of the optical fiber 116.
Japanese Examined Patent Publication (Kokai) No. 2-92838 discloses an optical fiber drawing furnace having a nozzle having a small diameter and a long length arranged immediately below a gas introduction portion provided at a lower portion of a furnace body and through which an optical fiber is drawn. The nozzle is provided to prevent entry of ambient gas into an inner space of the furnace body through the nozzle.
In practice, it is difficult to pass an optical fiber through such nozzle having a small diameter without contact when the drawing speed is high and the optical fiber vibrates in a transverse direction. In other words, such a optical fiber drawing furnace is not actually suitable to high speed drawing of the optical fiber. In addition to the above, a perfect seal against entry of ambient gas into the inner space of the furnace body cannot be achieved by such a nozzle without further reducing the diameter of the nozzle and further increasing the length of the nozzle. The narrower, longer nozzle may make production difficult and contact the drawn optical fiber.
Japanese Unexamined Utility Model Publication (Kokai) No. 59-153332, as shown in FIG. 3, discloses an optical fiber drawing furnace having two upper spaces 215 and 216 defined by two upper partitions 208 and 209 provided at an upper portion of a furnace body 201, through which an optical fiber preform 212 is introduced into an inner space 217, and a lower space 218 defined by lower partitions 210 and 211 provided at a bottom portion of the furnace body 201, through which an optical fiber 213 is drawn. In such optical fiber drawing furnace, inert gas is introduced into the inner space 217 through a lower gas introduction portion 204. The inert gas is also introduced into the lower-upper space 215 and the inner space 217 through an upper gas introduction portion 205. Further, inert gas is introduced into the lower space 218 through a lower gas introduction portion 206 to prevent entry of the ambient gas through a bottom hole provided in the lower partition 211. Inert gas is also introduced into the upper-upper space 216 through an upper gas introduction portion 207 to prevent entry of the ambient gas through a top hole provided in the upper partition 208.
The optical fiber drawing furnace disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 59-153332 discharges a large amount of inert gas into the atmosphere from the top and the bottom, therefore is uneconomical and contaminates the atmosphere. Further, such an optical fiber drawing furnace has a complex structure, is expensive, and requires difficult and troublesome maintenance work, therefore the production costs of the optical fiber become higher.
Japanese Examined Patent Publication (Kokoku) No. 7-84333 discloses an optical fiber drawing furnace wherein inert gas is introduced into an inner space of a furnace body at a top through which an optical fiber preform is inserted and the introduced inert gas is exhausted from a nozzle through which an optical fiber is drawn. The nozzle is provided with a suction chamber for adjusting an amount of gas to be exhausted through the nozzle to prevent entry of the ambient gas into the inner space of the furnace body.
The optical fiber drawing furnace disclosed in Japanese Examined Patent Publication (Kokoku) No. 7-84333 is aimed at the prevention of the contact of dust contained in the gas in the inner space to the optical fiber at the bottom nozzle through which the gas is exhausted so as to prevent the deterioration of the characteristics of the optical fiber. If the amount of the gas flowing from the inner space to the suction chamber is increased, the contact of dust and the optical fiber is increased and causes the deterioration of the characteristics of the optical fiber. There is a limitation to the increase of the suction gas flow, thus complete prevention of the entry of the ambient gas to the inner space is difficult.
The change of or variations in the diameter of the optical fiber preform may change the inner pressure of the inner space, but Japanese Examined Patent Publication (Kokoku) No. 7-84333 does not disclose or suggest any means of dealing with this. Such changes or variations may cause new disadvantages.
Japanese Unexamined Patent Publication (Kokai) No. 57-140330 discloses the idea of generating a gas flow at an upper portion and/or a lower portion in an optical fiber drawing furnace to prevent entry of the ambient gas into the inner space of the optical fiber drawing furnace. Japanese Unexamined Patent Publication (Kokai) No. 57-140330, however, merely discloses a technological idea and does not disclose or suggest control for determining a suitable value of sealing gas against variations in the inner pressure of the inner space due to the variations in a diameter of an optical fiber preform and/or variations in the drawing speed of an optical fiber. Accordingly, the technological idea disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-140330 cannot enable the generation of a gas flow for always preventing entry of the ambient gas into the inner space of the furnace body.